Multi-band antenna arrays, which can include multiple radiating elements with different operating frequencies, may be used in wireless voice and data communications. For example, common frequency bands for GSM services include GSM900 and GSM1800. A low-band of frequencies in a multi-band antenna may include a GSM900 band, which operates at 880-960 MHz. The low-band may also include Digital Dividend spectrum, which operates at 790-862 MHz. Further, the low-band may also cover the 700 MHz spectrum at 694-793 MHz.
A high-band of a multi-band antenna may include a GSM1800 band, which operates in the frequency range of 1710-1880 MHz. A high-band may also include, for example, the UMTS band, which operates at 1920-2170 MHz. Additional bands may comprise LTE2.6, which operates at 2.5-2.7 GHz and WiMax, which operates at 3.4-3.8 GHz.
A dipole antenna may be employed as a radiating element, and may be designed such that its first resonant frequency is in the desired frequency band. To achieve this, each of the dipole arms may be about one quarter wavelength, and the two dipole arms together are about one half the wavelength of the desired band. These are referred to as “half-wave” dipoles, and may have relatively low impedance.
However, multi-band antenna arrays may involve implementation difficulties, for example, due to interference among the radiating elements for the different bands. In particular, the radiation patterns for a lower frequency band can be distorted by resonances that develop in radiating elements that are designed to radiate at a higher frequency band, typically 2 to 3 times higher in frequency. For example, the GSM1800 band is approximately twice the frequency of the GSM900 band. As such, the introduction of an additional radiating element having an operating frequency range different from the existing radiating elements in the array may cause distortion with the existing radiating elements.
There are two modes of distortion that are typically seen, Common Mode resonance and Differential Mode resonance. Common Mode (CM) resonance can occur when the entire higher band radiating structure resonates as if it were a one quarter wave monopole. Since the stalk or vertical structure of the radiator is often one quarter wavelength long at the higher band frequency and the dipole arms are also one quarter wavelength long at the higher band frequency, this total structure may be roughly one half wavelength long at the higher band frequency. Where the higher band is about double the frequency of the lower band, because wavelength is inversely proportional to frequency, the total high-band structure may be roughly one quarter wavelength long at a lower band frequency. Differential mode resonance may occur when each half of the dipole structure, or two halves of orthogonally-polarized higher frequency radiating elements, resonate against one another.
One approach for reducing CM resonance may involve adjusting the dimensions of the higher band radiator such that the CM resonance is moved either above or below the lower band operating range. For example, one proposed method for retuning the CM resonance is to use a “moat,” described for example in U.S. patent application Ser. No. 14/479,102, the disclosure of which is incorporated by reference. A hole can be cut into the reflector around the vertical structure of the radiating element (the “feed board”). A conductive well may be inserted into the hole, and the feed board may be extended to the bottom of the well. This can lengthen the feed board, which may move the CM resonance lower and out of band, while at the same time keeping the dipole arms approximately one quarter wavelength above the reflector. This approach, however, may entail greater complexity and manufacturing cost.
In addition, a trade-off may exist between performance and spacing of the radiating elements in a multi-band antenna array. In particular, while array length may be used to achieve a desired beamwidth, it may be advantageous to reduce the number of radiating elements along the array length to reduce costs. However, reducing the number of radiating elements along the array length may result in increased spacing between the radiating elements, which may result in undesired grating lobes and/or attenuation.